The engineers defended their work to a committee appointed by President Barack Obama to review what's planned once the current shuttle program is retired.

The head of the office that has spent four years designing the next U.S. rocket, called Ares, told members of NASA's Human Space Flight Plans Committee that the current design was the safest, fastest way to get Americans back to space.

"We have done what we said we would do, and we are well on the way to our first test flight," said Steve Cook, head of the Ares project office at NASA's Marshall Space Flight Center in Huntsville.

Speaking during a public hearing, Cook dismissed suggestions by some that the space agency was on a flawed path with Ares.

"We are not drinking our own bath water," he said. "There have been several outside reviews since we began."

Other managers told the panel they were working through technical challenges with Ares, including a slim possibility that powerful energy waves created during a launch could injure astronauts or make it impossible for them to perform basic tasks, like looking at monitors.

The chairman of the review committee, longtime aerospace executive Norman Augustine, said members would offer broad options to Obama. Those could range from continuing to fly the space shuttle to moving forward with Constellation without any changes, he said.

"We will not be in the tweaking business," Augustine said during a news conference.

Under a $35 billion plan put in place under former President George W. Bush, NASA is working to retire the shuttle fleet by the end of 2010 and return to human flight no sooner than 2015 aboard Ares.

Obama appointed the Augustine committee in May to seek information from NASA, outside scientists, the aerospace industry and Congress on the best way to send astronauts into space. It will submit a final report on Aug. 31, Augustine said.

Engineers are still working on the Constellation program during the review, and NASA plans to launch a test version of the human-carrying Ares I rocket by Oct. 31.

Work isn't as far along on a larger rocket called Ares V, which would lift heavy equipment into orbit for a moon mission and an eventual trip to Mars.

Former NASA Administrator Michael Griffin met with the panel privately before the public hearing. In a letter to members, Griffin said the Constellation program was being subjected to "broad but shallow criticism" when NASA needs continuity in its planning.

"Do not allow the parochial voices of the small-minded, the self-interested, and the uninformed to prevail," wrote Griffin, who now teaches at the University of Alabama in Huntsville. "Choose the future."

Arguing that Augustine's committee wouldn't even exist without budget concerns, Griffin said U.S. spaceflight shouldn't be done "on the cheap."